A hydrostatic drive is known which comprises a variable-displacement pump permanently connected to the output shaft of an internal-combustion engine and connected via a pair of conduits to a hydraulic motor whose output in turn serves to power the vehicle. Normally such an arrangement is used to displace the vehicle along the ground and also to operate the various peripheral equipment of the vehicle.
The variable-displacement pump is normally constituted as an axial-piston pump and the control element is the swash plate thereof. In a neutral position of this swash plate, that is perpendicular to the rotation axis, the pump, even though driven, does not displace any fluid. When tipped to one side of this position the motor connected to this pump will be driven in one direction, and when tipped to the other side of this position the motor will be driven in the opposite direction. Thus such a system allows a diesel engine which only operates at peak efficiency in a so-called nominal range to be used to drive a shaft in either direction at any of a multiplicity of speeds. The engine is simply set to operate at its nominal speed and the steplessly variable transmission couples it to the output.
Normally a servomechanism, typically formed as a double-acting hydraulic cylinder, is employed to displace the control element. In such a system a so-called control pump is connected to the engine along with the variable-displacement pump, so that when the control element of the variable-displacement pump is in the neutral position this control pump will be able to produce a high pressure suitable for operating the servocylinder. Typically a four-port three-position reversing valve is provided connected on one side to the two chambers of the double-acting cylinder and on the other side to the high-pressure side of the control pump and to the sump. Thus displacement of this valve into one end position will displace the control element in one direction from the neutral position and displacement in the opposite direction will displace the control element similarly oppositely.
It has further been suggested in Technical Report 1/73 of BRUENING HYDRAULIK GmbH entitled "HYDROSTATISCHE ANTRIEBE" to automatically limit the pressure in the system with increasing operation speed. Such a system prevents a predetermined maximum torque limit from being exceeded. Nonetheless this system has the considerable disadvantage that the drive engine is slowed down somewhat at all speeds, with the amount of wasteful slowing down being greatest at the higher output levels for the drive system.
It has further been suggested from the report "ZF-CONTROLS FOR HYDROSTATIC TRANSMISSIONS" of Zahnradfabrik Friedrichshafen AG (F 11/6 ZFF 77304) to reduce the effect of the control pressure on the control piston of the regulating device by a force proportional to the working pressure, in conjunction with a corresponding reduction of the drive engine's RPM. Thus in the nominal or central drive pressures the reduction in RPM is at a maximum whereas in a condition wherein, for instance, the vehicle is pushing an immovable object, it is possible to operate with maximum working pressure. Such a system, therefore, also slows down the drive engine at the critical intermediate range, wasting valuable energy.